This invention relates generally to magnetic resonance imaging of an object having different chemical species therein, such as fat and water, and more particularly the invention relates to species imaging in the presence of magnetic field heterogeneity.
The ability to image different chemical species such as fat and water using magnetic resonance techniques is particularly important in medical applications. For example, imaging and diagnosis of articular cartilage abnormalities has become increasingly important in the setting of an aging population where osteoarthritis is second only to cardiovascular disease as a cause of chronic disability. Accurate assessment of articular cartilage is also essential with the advent of surgical and pharmacological therapies that require advanced imaging techniques for initial diagnosis and management of disease progression.
Ideal imaging of articular cartilage requires high resolution and good contrast with adjacent tissues; this can be markedly improved with fat suppression techniques. In addition, bright appearance of synovial fluid is advantageous as it provides an arthroscopic effect and “fills in” defects in articular cartilage, increasing the conspicuity of cartilage irregularities. Separating fat and water can increase the conspicuity of the both the water (for most applications) and fat (for special applications) with many types of pulse sequences and with both T1 and T2 weighted sequences.
The difficulty in decomposing different chemical species is compounded by the presence of magnetic field heterogeneity. Separation of fat and water through “in-phase” and “out-of-phase” imaging is an approach first demonstrated by Dixon, Radiology 1984; 153: 189-194, and further refined by Glover, Journal of Magnetic Resonance Imaging 1991; 1:521-530, to compensate for the effects of magnetic field heterogeneities. In Glover's work, a three-point sampling scheme that acquires spin-echo or gradient echo images with echo time (TE) increments of 0, 2.2, and 4.4 ms, and produce phase increments of 0, π, and 2π, when the frequency difference between fat and water is approximately −220 Hz at 1.5 T. The mathematics for this special case are greatly simplified and post-processing calculations are faster; however, these values of TE lengthen the minimum TR and would cause severe image degradation with SSFP imaging, for example, in the presence of typical magnetic field heterogeneities. Application of “Dixon” imaging to fast spin-echo (FSE) sequences has also been limited because the acquisition of echoes at different time shifts with respect to the spin-echo increases the spacing between successive refocusing pulses (echo spacing). Increasing the echo spacing reduces the number of echoes that can be collected in a time that maintains acceptable blurring from T2 decay, offsetting the scan time benefits of FSE. A fat-water separation method that permitted shorter time increments would reduce the time between refocusing pulses and be beneficial to fast spin-echo imaging.
SSFP is a rapid gradient echo imaging technique with renewed interest in recent years, owing to widespread availability of high speed gradient systems. SSFP has superior signal to noise ratio (SNR) compared to other gradient echo techniques and has excellent contrast behavior that has mixed dependence on T1 and T2. In particular, synovial fluid appears bright on SSFP images owing to its long T2. The major limitation of SSFP is severe image degradation caused by local magnetic field heterogeneities if the repetition time (TR) is long.